1. Field of the Invention
The present invention relates to microwave resonators and more particularly to microwave resonators that may be used for acceleration detection, and to a method for compensating such resonators for cavity dimension changes that can be caused by factors such as temperature variation.
2. Description of the Prior Art
Microwave resonators have long been known in the art. Typically they comprise a cavity formed, for example by a hollow cylindrical member, usually circular in cross-section although other cross-sectional shapes have been used, closed at both ends and with a high frequency oscillator connected through a suitable connector at one end of the cavity to radiate radio frequency energy therein. A second connector in the cavity receives a portion of the energy Which is conducted to a peak detector. At certain frequencies, a standing wave known as a "transverse electric resonant peak" is established and the peak detector shows a maximum value. The frequency at which such standing waves are formed depends on the length and the cross-sectional area of the cavity. Such a cavity may be used to sense acceleration by making one end movable under the force of acceleration so as to change the cavity length and thus the standing wave. Such a prior art accelerometer can be as seen in FIG. 1. In FIG. 1, a container 10 is formed from a hollow cylinder 12 having a bottom end 14 and a top lend 16. Container 12 produces a cavity 18 with a length Z and a cross-sectional area represented by diameter A (for a circular cross-section). An RF oscillator 20 produces a signal on a conductor 22 connected to a connector 24 extending through the bottom end 14 so as to radiate energy into cavity 18. The radiated energy is received through a connector 26 and, by a conductor 28, is presented to a suitable peak detector 30. The output of the peak detector is connected back to oscillator 20 by a conductor 32. The frequency produced by the oscillator 20 is changed until the peak detector detects a peak indicating that a standing wave has been established in the cavity 18. The signal from the peak detector on conductor 32 stops the frequency change of the oscillator 20 and the frequency causing the standing wave is presented from oscillator 20 to output circuitry 36, such as a microprocessor, via a conductor 38.
The second end 16 of container 10 is arranged to be movable, up and down, as shown by double headed arrow 40 along the inner surface of cylinder 12, and is fastened by a spring 44 to a base member 42 which is rigidly connected to cylinder 12. If the container 10 is subjected to acceleration up or down in either direction of arrow 40, upper end 16 will move soi as to increase or decrease the length Z of cavity 18. The change in length Z causes a change in the frequency at which the standing wave is established and the peak detector 30 will again operate to cause oscillator 20 to change frequency until the standing wave is reestablished. The frequency of the new peak compared to the frequency of the old peak is related to the change in Z and thus to the acceleration experienced by the container 10. The output circuitry operates on the two frequencies received on conductor 38 to produce an output shown by arrow 46 indicative of acceleration.
The apparatus described in connection with FIG. 1 will operate satisfactorily so long as the only cavity dimension change is that due to acceleration. Unfortunately, however, other factors such as temperature changes can also affect the cavity dimensions and would cause false acceleration indications if left uncompensated.
There have been several suggestions for compensating for temperature proposed in the prior art. For example, as shown in FIG. 1, a temperature sensor 47 can be mounted on cylinder 12 and used to measure any temperature changes and to provide an input indicative thereof on a line 48 to the output circuitry 36 which would then correct the output on line 46 in accordance therewith. This is not practical for very accurate systems since temperature measurements to 1/100 of a degree centigrade are necessary and variations of temperature greater than this may be found in different parts of the cavity. Thus detecting the temperature variation at any location in the cavity may not provide information necessary to determine the change in cavity dimension since other parts of the cavity may be subjected to different temperature variations. Another temperature compensation technique has been suggested in the prior art. A second container having a second cavity therein and with second oscillators, peak detectors etc. is mounted on top of the first container so that as the upper end 16 of FIG. 1 forms the lower end of the second cavity. Thus as upper end wall 16 moves to increase or decrease the length Z in FIG. 1, it will act oppositely to decrease or increase the length of the second cavity in the second container. The outputs of the two cavities are combined to produce the indication of acceleration (the effect of acceleration on the two cavities is opposite, i.e. one standing wave frequency will increase while the other decreases) but both are indicative of the acceleration. However, if temperature changes effect both cavities in the same way, the effect of temperature on cavity dimension is nullified. While this system should operate satisfactorily in low to medium accuracy acceleration measurement systems, the need for precise temperature uniformity between the two cavities or the precise measurement of each cavity make this impractical for high precision application. Furthermore, the requirement for two cavities and all the associated apparatus and electronics is burdensome from a space and cost standpoint.